Silicon controlled rectifier dimmers are generally used by common incandescent lamps and halogen lamps for dimming. Silicon controlled rectifier dimmers alter the effective current value of the AC current through changing the conduction angle, so as to dim the incandescent lamps and halogen lamps.
FIG. 1 shows a block diagram of an exemplary prior-art silicon controlled rectifier dimmer 227. In such a silicon controlled rectifier dimmer 227, a RC network comprising a resistor 222, an adjustable resistor 223 and a capacitor 225 can delay the conducting duration of the silicon controlled rectifier 226 until the capacitor 225 is charged and voltage Vtriac 226 reaches a triggering voltage value. Then the silicon controlled rectifier 226 is turned on, so the silicon controlled rectifier dimmer 227 works, and the perfect operation wave of which is shown in FIG. 2. Wherein, during a time duration from moment t0 to moment t1, the silicon controlled rectifier 226 is turned off, which results in an open circuit. So voltage between the two terminals of load 221 is 0. When voltage of capacitor 225 is charged to a certain value (at moment t1), the silicon controlled rectifier 226 is turned on, which results in a short circuit. So voltage between the two terminals of load 221 is equal to Vac. In a half cycle period from moment t0 to moment t2, during a time duration from moment t1 to moment t2, the silicon controlled rectifier 226 is turned on and the resistance of adjustable resistor 223 is adjusted to change the duration of the on time for dimming.
According to the prior-art dimming method, only one silicon controlled rectifier dimmer 227 is required, and no other circuits and elements should be added. The advantage of using silicon controlled rectifier dimmer 227 is its simplicity of circuit implementation. In additional, changing the conduction angle directly will not affect power factor of resistive light emitters such as incandescent lamps and halogen lamps. Accordingly, dimming by a silicon controlled rectifier dimmer can be widely used in the illumination of incandescent lamps.
FIG. 3 shows a functional block diagram of an exemplary prior-art LED driving circuit. As shown in FIG. 3, LED driving chip 215 senses the primary current information at CS terminal, and transforms the primary current information into a voltage Vfb which is proportional to the secondary current via secondary current sampling module 237. Voltage Vfb reaches reference voltage Vref via a negative feedback clamping, so as to drive LED with a constant current. Accordingly, to some extent, reference voltage Vref decides the lighteness of the LEDs.
Many LED illuminations employ silicon controlled rectifier dimmer for dimming so as to be compatible with existing illumination application circumstance. However, problems as follows present in LED illuminations when using silicon controlled rectifier dimmer for dimming.
1. Sine waves of input current are destroyed by dimming via a silicon controlled rectifier dimmer, and power factor is also reduced. Normally, power factor of the LED driving circuit employs silicon controlled rectifier dimmer for dimming is lower than 0.5. Moreover, the smaller the conducting angle, the worse the power factor.
2. The nonsine waves increase harmonic coefficient while reducing the power factor.
3. The nonsine waves generate serious interference signals in the circuit.
4. Dimming via a silicon controlled rectifier dimmer is unstable for a small load such as a LED, and requires a bleeder resistor consuming additional power and reducing efficiency.
To sum up, a LED dimming device being compatible with existing illumination application circumstance and possessing high efficiency is highly demanded.